Pressure Drop, Natural Refrigerant, R744
For the design of air conditioning systems the automotive industry often uses the AirConditioning Library written in the modeling language Modelica. In this one-dimensional modeling approach system components are discretized only in flow direction. The refrigerant pipes connecting components like compressor, evaporator and condenser are usually modeled as straight pipes with equivalent length or they are neglected. Due to the arrangement of components in the engine compartment refrigerant pipes contain a large number of bends, contractions and expansions as well as corrugated tubes. This leads to a considerably higher pressure drop of these pipes compared to straight pipes. On the other hand pipe cross section should be minimized to keep the filling volume of the refrigerating unit as small as possible. Hence, the aim of the study conducted at the Institute of Engineering Thermodynamics is to investigate experimentally and numerically the pressure drop of refrigerant pipes of an R744 automotive air conditioning system to improve pipe models used in the system simulation. This paper presents a test rig to measure the pressure drop of arbitrary shaped refrigerant pipes. The experimental layout is basically a pump circuit using the refrigerant R744. The system pressure can be adjusted to values between 30bar and 140bar. Temperature at the inlet of the test section can be adjusted between -10°C and +130°C and mass flow rate can be set up to 250kg/h. Thus, the test rig covers the complete operating range of an R744 automotive air conditioning system. Besides the experimental investigations, results of a CFD simulation are presented. With Star-CCM+ one-phase flow and two-phase flow through the pipes is simulated and the resulting pressure drop is compared with experimental values. Validation experiments have been performed with a horizontal straight pipe made of stainless steel with an inner diameter of 8mm and a length of 3.2m. There is a good agreement between experiments, CFD results and the pressure drop calculated with the well-known Colebrook-White correlation. For arbitrary shaped pipes without corrugated tubes CFD results and experimental values are in good agreement. For corrugated tubes CFD generally predicts a lower pressure drop compared to experimental investigations. Conclusively, the impact of pressure drop in refrigerant pipes on system behavior is investigated numerically through a comparison of different adapted pipe models in a standard R744 air conditioning system model.